1. Field of the Invention
The present invention relates to an adjusting method and device for semiconductor pressure switches.
2. Description of the Related Art
Manufacturing methods for conventional semiconductor pressure switches will be explained referring to the flowcharts shown in FIGS. 3(a) to 3(l) and FIGS. 4(a) to 4(d).
First, a photoresist layer is coated on the surface of a 525 .mu.m thick, N-type silicon substrate 1, which is subjected to a light exposure process, developed, and then patterned so as to remove a portion of the photoresist layer resulting in a recessed portion 2 (refer to FIG. 3(a)). After the photoresist patterning process, the silicon substrate 1 is placed in a plasma etching system. A mixture of CF.sub.4 and O.sub.2 is introduced in the system, and by applying a high frequency of about 70 W, the silicon substrate 1 is etched selectively to form a recessed portion 2 with a depth of about 3 .mu.m (refer to FIG. 3(b)). After the formation of the recessed portion 2, a photoresist layer is re-coated on the surface of said silicon substrate 1, and the photoresist layer is exposed to light and developed. The portion of the photoresist layer where a boron-doped electrode 3 is to be formed is patterned by removing. Next, a boron-doped electrode 3 is formed by ion-implanting boron atoms in the system (FIGS. 3(c) and 3(d). After the boron-doped electrode formation, an oxide film 4 acting as an insulating film is formed and then patterned selectively to leave said oxide film in said recessed portion 2 (refer to FIGS. 3(e) and 3(f)). Successively, a polycrystalline silicon film 5 is formed on the upper surface of said oxide film 4 using an LPCVD process. The film 5 is selectively etched using a mixture of a hydrofluoric acid and a nitric acid so as to leave the polycrystalline silicon film 5 at a portion where a contact electrode 6 is formed (refer to FIGS. 3(g), 3(h) and 3(i)).
Next, in order to form a contact electrode 6, a wiring electrode 7, and a bonding pad 8, Au and Cr films are formed sequentially on the oxide film 4 and the boron electrode 3 using sputtering. Then, the Au and Cr are etched selectively to form the contact electrode 6, the wiring electrode 7, and the bonding pad 8 (FIGS. 3(i) and (j)). Au and Cr are then sputtered on a surface of a glass substrate 9 and patterned to form a reference electrode 10 (refer to FIGS. 3(k) and 3(l)).
Next, the glass substrate 9 and the silicon substrate 1 are aligned so as to face the contact electrode 6 with the reference electrode 10 as shown in FIG. 4(a). The structure is then placed on a heater 15 (FIG. 2). Thereafter, the structure is heated at about 400.degree. C. while the silicon substrate 1 and the glass substrate 9 are anodic-welded to each other by applying 0 V to the glass substrate and about 500 V to the silicon substrate 1 for about 20 minutes (refer to 4(a)). After the anodic welding, a silicon nitride film 14 is formed on the back surface of the silicon substrate 1 being the opposite side with respect to the welded surface of the glass substrate 9. The silicon nitride film 4 is then patterned selectively using a phosphoric acid at 150.degree. C. to form a diaphragm 11 in the silicon substrate 1 (refer to FIG. 4(b)). Furthermore, an alkali-proof coating material 16 is coated on the bonding pad 8 overlaying the silicon substrate 1 and the silicon substrate 1 is then immersed into a potassium hydroxide solution at 90.degree. C. for about 3.5 hours. As a result, the silicon substrate 1 is etched back to about 500 .mu.m to form a diaphragm 11 of 22 .mu.m thick (FIG. 4(c). A desired pressure switch is then produced by removing the coating material 16 (FIG. 4(d)).
Next, another conventional semiconductor pressure switch manufacturing method will be explained referring to flowcharts shown in FIGS. 5, 6 and 7.
First, a photoresist layer is coated on the surface of a 525 .mu.m thick, N-type silicon substrate 1 which is subjected to a light exposure process, is developed, and then is patterned to form a recessed portion 2. After the photoresist patterning process, the silicon substrate 1 is placed in a plasma etching system. A mixture of CF.sub.4 and O.sub.2 is introduced in the system and by applying a high frequency of about 70 W the silicon substrate 1 is etched selectively to form the recessed portion 2 with a depth of about 3 .mu.m (refer to FIG. 5(a) and 5(b)).
After the formation of the recessed portion 2, a photoresist layer is coated on the surface of the silicon substrate 1 which is exposed, developed and patterned before forming a boron electrode 3. Then, a boron electrode 3 is formed by ion-implanting boron atoms in the system (FIGS. 5(c) and 5(d)). After the formation of the boron electrode 3, an oxide film 4 acting as an insulating film is formed and then patterned selectively so that the oxide film remains in the recessed portion 2 (refer to FIGS. 5(e) and 5(f)).
Successively, a polycrystalline silicon film 5 is formed using an LPCVD process. The film 5 is selectively etched using a mixture of a hydrofluoric acid and a nitric acid so as to leave the polycrystalline silicon film 5 on a portion of the oxide film 4 and a portion of the boron electrode 3 where a contact electrode 6 is formed (refer to FIGS. 6(a), 6(b) and 6(c)).
Next, in order to from a contact electrode 6, a wiring electrode 7, and a bonding pad 8, Au and Cr films are formed sequentially on the oxide film 4 and the boron electrode 3 using sputtering. Then Au and Cr are etched selectively to form plural contact electrodes 6, plural wiring electrodes 7, plural fuses 12 connecting the wiring electrodes to the boron electrodes, and a bonding pad 8. (FIGS. 6(c) and (d)).
Au and Cr are then sputtered on a surface of a glass substrate 9 and patterned to form a reference electrode 10 (refer to FIGS. 7(a) and 7(b)).
Next, the glass substrate 9 and the silicon substrate 1 are arranged on a heater 15 so as to face the contact electrode 6 with the reference electrode 10 (FIG. 7(c)). The structure is heated at about 400.degree. C. while the silicon substrate 1 and the glass substrate 9 are anodic-welded to each other by applying 0 V to the glass substrate 9 and about 500 V to the silicon substrate 1 for about 20 minutes. After the anodic welding, a silicon nitride film 14 is formed on the back surface of the silicon substrate 1 being the opposite side with respect to the welded surface of the glass substrate 9. The silicon nitride film 4 is then patterned selectively using a phosphoric acid at 150.degree. C. to form a diaphragm 11 on the silicon substrate 1 (FIG. 7(d)).
Furthermore, an alkali-resistant coating material 16 is coated on the bonding pad 8 formed on the silicon substrate 1 and immersed into a potassium hydroxide solution at 90.degree. C. for about 3.5 hours to etch the silicon substrate 1 to about 500 .mu.m to form a diaphragm 11 of 22 .mu.m thick (FIG. 7(e)). A pressure switch is then formed by removing the coating material 16 (FIG. 7(f)).
Successively, in order to perform switching at a desired pressure switching, the manufactured switch is arranged in a pressure chamber 13 and then pressurized at a pressure of 1.95 kg/cm.sup.2 which is below a desired pressure of 2 kg/cm.sup.2. A voltage of about 5 V is applied to the pressure chamber 13 hermetically sealed by way of lead wires (FIG. 7(g)). A pressure switch which can operate at a desired pressure is manufactured by destroying some fuses 12 which are in contact with the contact electrodes 6 under a pressure of below 1.95 kg/cm.sup.2 and by contacting the remaining contact electrodes 6 at a pressure of more than 2 kg/cm.sup.2 (refer to FIG. 7(h)).
However, according to the conventional manufacturing method, it is difficult for the semiconductor pressure switch to perform a switching operation at a desired pressure because of the variations in the etching depth of the recessed portion, the height of the contact electrode formed within the recessed portion, and the thickness of the diaphragm, thus resulting in larger switching error to pressure and bad manufacturing yield.
According to the conventional fuse trimming art, although it is possible to manufacture a pressure switch which can operate somehow under a desired pressure when an accuracy rating within about .+-.0.1 kg/cm.sup.2 is necessary, the spacing between adjacent individual contact electrodes must be less than 2 .mu.m if the spacing of the neighboring contact electrodes is trimmed for a pressure accuracy of 0.1 kg/cm.sup.2. Therefore, the contact electrode has to be less than 1 .mu.m at maximum in size which makes it impossible to form the contact electrode in the 3 .mu.m recessed portion. Hence, there has been a problem of difficulty in achieving good pressure accuracy.